Evaluation of an Integrated Upright CT In a First-of-Its-Kind Upright Proton Therapy System
Abstract
Purpose
Upright radiotherapy offers physiological and dosimetric advantages. Mevion S250-FIT, world’s first clinical ultra-compact synchrocyclotron-based upright proton therapy system, incorporates a novel upright CT for treatment planning and image guidance. This study presents performance evaluation of the first clinical installation of this novel system in comparison with a state-of-the-art diagnostic CT.
Methods
CT imaging performance was evaluated on image quality, imaging dose, and Hounsfield-unit (HU)-to-stopping-power-ratio (SPR) calibration. Scans were acquired using both the upright and a state-of-the-art diagnostic CT (Siemens Biograph-mCT). Imaging dose for each system was quantified using standard CTDI phantom measurement protocols. Image quality was evaluated using a Catphan 504 phantom, with analysis of geometric distortion, spatial resolution, uniformity, high/low contrast, noise, and HU accuracy using SunCHECK. HU-SPR table was established using CIRS 062M Phantom with tissue-equivalent inserts, following established consensus guidelines. For each insert, HU values were extracted from CT images, and their corresponding SPRs were calculated theoretically based on elemental composition and mass density. A curve was subsequently fitted between HU and SPR.
Results
The upright CT demonstrated image quality comparable to that of a diagnostic CT regarding geometric distortion, noise, spatial resolution, and uniformity. Measured CT numbers satisfied specification requirements for most materials, with deviations observed for air, Delrin (338.5 HU), and Teflon (903.9 HU). Low-contrast detectability was inferior to diagnostic CT. Imaging doses for the upright CT were 10.5 mGy (abdomen) and 21.5 mGy (head), compared with 12.7 mGy (abdomen) and 57.7 mGy (head) for diagnostic CT. Differences in SPR values derived from upright CT-based and diagnostic CT-based calibrations were marginal (0.00016±0.0036, corresponding to a relative difference of 0.12%±0.68%) across the phantom.
Conclusion
The upright CT integrated into Mevion S250-FIT system demonstrates satisfactory performance in both image quality and HU-to-SPR calibration when compared with state-of-the-art diagnostic CT.